What elements go into a good material testing scene
Make sure you have correct neutral lighting (a diffuse grey (50%)
sphere should render as diffuse grey (50%)).  
You need some reflectors to judge reflections.
Maybe you should have an object with a stepped thickness so that you
can see how refraction behaves.
You could also setup additional renderlayers with different lighting
situations. 
Maybe a grid on the floor to get the scale right (cm, dm, m)?
A McBeth chart of some sort?
A diffuse and a reflective sphere for reference in some corner?


I think SHED have some nice examples in their blog somewhere...